A typical disc drive includes one or more magnetic discs mounted for rotation on a hub or spindle. A typical disc drive also includes a transducer supported by a hydrodynamic air bearing which flies above each magnetic disc. The transducer and the hydrodynamic air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the magnetic discs and to store information on the magnetic discs.
An electromechanical actuator operates within a negative feedback, closed-loop servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations.
Information is typically stored in concentric tracks on the surface of the magnetic discs. A plurality of axially aligned tracks on the disc surfaces are collectively referred to as a cylinder. Data is written to the discs by providing a write signal to one of the data heads to encode flux reversals on the surface of the magnetic disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the electromechanical actuator so that the data head flies above the magnetic disc, sensing the flux reversals on the magnetic disc, and generating a read signal based on those flux reversals. The read signal is typically conditioned and then decoded by the drive controller to recover data represented by flux reversals stored on the magnetic disc, and consequently represented in the read signal provided by the data head.
In an embedded servo-type system, servo information is recorded on tracks which also contain data stored on the disc drive. The servo data (or servo bursts) are written on the data tracks and are commonly evenly temporally spaced (or angularly spaced) about the circumference of each track. Data to be stored on the disc drive is written between the servo bursts. Also, some disc surfaces include header information in header fields which are written on the disc. Header fields correspond to the servo bursts, or may number less than the servo bursts (i.e., they may correspond to every other servo burst, and so on).
As a transducer reads the servo information, the transducer provides a position signal which is decoded by a position demodulator and presented in digital form to a servo control processor. The servo control processor compares the actual radial position of the transducer over the disc (as indicated by the embedded servo burst) with desired position and commands the actuator to move in order to minimize position error. In addition, when the host system requests that the disc drive access a new portion of the disc surfaces, the servo control processor controls the disc drive to move the plurality of data heads to an appropriate cylinder to begin accessing the designated disc surface.
In large data throughput environments, such as audio and video environments, random access of the large chunks of data associated with these environments presents significant problems. The two primary components which contribute to delays in accessing data from a disc drive include the seek time, and the rotational latency. The seek time is the time required for the disc drive to seek from one cylinder to another. The rotational latency is the time required for the disc surfaces, once the data heads have been positioned over the proper cylinder, to rotate to a point where the data head over the desired track reaches the beginning of the desired data and can commence accessing data from that track.
In the past, the delays associated with seek operations have been controlled by conventional mechanisms which reorder accesses to the disc drive in servicing the requests. Accesses are conventionally reordered in a sweeping fashion from one disc radius to the other (i.e., from the inner disc radius to the outer disc radius) in order to reduce the average length of a seek operation. For instance, if the actuator arm is currently positioned near the outer diameter of the disc, then the disc drive controller rearranges access requests provided to the drive such that the actuator arm progressively moves from the outer diameter toward the inner diameter of the drive. This type of scheduling is not unique to the present invention and has been around for many years.
However, rotational latency has not to date been addressed in any meaningful fashion. Under current systems, the disc surfaces must rotate a random amount of time (or angular distance) before the data head associated with the desired disc surface arrives at the beginning of the track (or at the beginning of the next chunk of data). The maximum amount of time which can be associated with such rotation is the time associated with one revolution of the disc. Thus, the worst case access time for data in such a system is equal to the time for the seek operation plus the time for one full rotation of the disc surfaces.
The present invention addresses these and other problems, and offers other advantages over conventional, prior art systems.